<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>事件重定向</title>
</head>
<body>
  <div onclick="console.log('Handled outside:', event.target)"></div>
  <script>
    document.querySelector('div')
      .attachShadow({ mode: 'open' })
      .innerHTML = `
        <button onclick="console.log('Handled inside:', event.target)">Foo</button>
      `;
  </script>
</body>
</html>